On Fri, 17 Aug 2007 02:13:06 GMT, Brian Whatcott
wrote:

On Thu, 16 Aug 2007 09:59:09 -0700, Frogwatch
wrote:

I will defend my suggestion although it sounds impractical for the
Alberg. A coupler properly made, probably from SS about 1" ID larger
than the shaft with deep holes drilled and then milled to size will be
as well balanced as the original. A few barnacles on a prop would
produce a far greater imbalance.
I have been amazed at how my 3 bladed prop does not shake even though
it gets covered with barnacles. I hit a mast from a dismasted Laser
this past year and made a huge ding in one blade and it still turns
smoothly. This causes me to believe that it is alignment, not balance
that is most critical and a well made coupler would maintain that
alignment. Besides, you can always re-align the shaft (fairly easy on
my S2).


Here's a little engineering insight. If a drive shaft is lightened
by drilling out an axial hole whose diameter is one half the outer
diameter - the shaft's strength in torque is reduced by 5% and its
weight is reduced by 25% This is important in aircraft
construction.

So as applied to cutting and shutting a propellor shaft, if its
diameter is x, and it is cut then bridged with a coupler,
the coupler's diameter ought to be just over twice the shaft's
diameter, to bridge across the gap. As to passing the torque into
and out of the coupler - the ideal connection is splines, the more the
merrier, within reason. But this is unlikely to be an option.
Another option, much simpler to arrange, is two well-fitted bolt holes
at right angles to each other, at one shaft diameter back from the
cut, and the other at two diameters back if possible. This is likely
to be weaker than the original torque capability.
There is another option, which will be familiar to you from
examining an old style (English) bike pedal crank.
A flat is worked on the shaft, and the pedal crank has a hole
to pass the shaft. But it also has a hole at right angles to the shaft
through which a cotter bolt with a tapered flat passes to bear on the
main shaft's flat. The cotter, when bolted tight, eliminates slippng
between the two pieces, so a wear point is eliminated.
If an eccentric hole can be drilled in the coupler, so a cotter bolt
can bear on the shaft, it would stop relative motion.
But there are many other ways you could concieve of connecting a shaft
into and out of a coupler, I'm sure.

Brian Whatcott Altus OK


I think the point you are missing is that although a coupling may very
well cope with the torque there just isn't any room to fit a coupling
of the type you describe at either the propeller or the engine end.
One might possibly use a larger cutlass bearing running on the O.D. of
the coupling but that would just add complications to a system that
should be as sim0le as possible.

The most logical answer is(1) remove the rudder, or (2) saw the cutout
in the rudder larger.




Bruce in Bangkok
(brucepaigeATgmailDOTcom)

On Fri, 17 Aug 2007 02:13:06 GMT, Brian Whatcott
wrote:

On Thu, 16 Aug 2007 09:59:09 -0700, Frogwatch
wrote:

I will defend my suggestion although it sounds impractical for the
Alberg. A coupler properly made...


Here's a little engineering insight. If a drive shaft is lightened
by drilling out an axial hole whose diameter is one half the outer
diameter - the shaft's strength in torque is reduced by 5% and its
weight is reduced by 25% This is important in aircraft
construction.

So as applied to cutting and shutting a propellor shaft, if its
diameter is x, and it is cut then bridged with a coupler,
the coupler's diameter ought to be just over twice the shaft's
diameter, to bridge across the gap.
....
Brian Whatcott Altus OK


I knew, sure as sure, as soon as I wrote "engineering insight" there
would be a screw-up. And there was.
If you want to maintain torque capacity though a coupling, it doesn't
need to be TWICE the shaft diameter. That's three times the cross
section area, at about twice the distance from the axis.
The coupler diameter doesn't need to be even 1.5 times the diameter
of the shaft. 1.3 times the shaft diameter would do it.
So a coupler whose diameter is about one inch bigger than the shaft
could handle up to a three inch shaft, if well connected. Yeah Baby!

Brian W

Jonathan W wrote:
I bought an Indigo 3 bladed prop last week for my Alberg 35 with the
original Atomic 4, as the existing prop succumbed to electrolysis

I went down to the boat yesterday, worried that I might not be able to
loosen the old hub's grip on the shaft. It was also a 3 bladed prop.
That was actually easy after about 5 minutes with a map gas torch.

I seem to have only about an inch of space between the end of the prop
shaft and the rudder (post). The hub is about 2-2,5 inches thick. I've
tried swinging the rudder from stop to stop, but don't seem to be able
to find an angle that will allow the old prop to come off, or, for that
matter, a new one to go on.

Others must have encountered this problem and found a solution.....

At this juncture it seems to me that I either have to dismount the
rudder to gain an extra inch or so, or I have to unmount the engine and
pull the prop shaft forward a couple of inches (if that's even possible )

I am hoping that I missed something perfectly obvious as I was working
late and was tired.... any help appreciated.

Thanks,

Jonathan Wye



Well, the m marina figured it out. Apparently with enough patience one
could align the prop and rudder and "turn" it off. I had thought
something like this might be the case, as I was tired and it was late
when I got to the prop last weekend.

Also, some years ago I got hit by a Mack truck (literally) and sometimes
I have trouble with certain 3 dimensional space/configuration problems.
This might have been a bit of that, too.

In any event, the new prop is on, and the boat is back in the water.
Wish I could have gotten down this weekend to try it out.

thanks for all the interesting commentary

Jonathan

--
I am building my daughter an Argie 10 sailing dinghy, check it out:
http://home.comcast.net/~jonsailr

Jonathan W wrote:
Jonathan W wrote:

I bought an Indigo 3 bladed prop last week for my Alberg 35 with the
original Atomic 4, as the existing prop succumbed to electrolysis

I went down to the boat yesterday, worried that I might not be able to
loosen the old hub's grip on the shaft. It was also a 3 bladed prop.
That was actually easy after about 5 minutes with a map gas torch.

I seem to have only about an inch of space between the end of the prop
shaft and the rudder (post). The hub is about 2-2,5 inches thick.
I've tried swinging the rudder from stop to stop, but don't seem to be
able to find an angle that will allow the old prop to come off, or,
for that matter, a new one to go on.

Others must have encountered this problem and found a solution.....

At this juncture it seems to me that I either have to dismount the
rudder to gain an extra inch or so, or I have to unmount the engine
and pull the prop shaft forward a couple of inches (if that's even
possible )

I am hoping that I missed something perfectly obvious as I was working
late and was tired.... any help appreciated.

Thanks,

Jonathan Wye



Well, the m marina figured it out. Apparently with enough patience one
could align the prop and rudder and "turn" it off. I had thought
something like this might be the case, as I was tired and it was late
when I got to the prop last weekend.

Also, some years ago I got hit by a Mack truck (literally) and sometimes
I have trouble with certain 3 dimensional space/configuration problems.
This might have been a bit of that, too.

In any event, the new prop is on, and the boat is back in the water.
Wish I could have gotten down this weekend to try it out.

thanks for all the interesting commentary

Jonathan

I'm glad you found a solution. Having said that, what year was all of
this assembled? Maybe removing the rudder, looking for corrosion on the
pieces and giving everything as good cleaning and greasing would be a
good idea.